The invention relates to new and useful improvements in electrical power supplies. More particularly, the invention relates to an electrical power supply with a low-loss inrush current limiter and a step-up converter circuit.
Electrical power supply units require special precautions to minimize the loads on the power supply grid caused when the units are switched on.
Patent Abstracts of Japan E-1387, Jun. 24, 1993, Vol. 17/No. 334, 5-38139 (A), discloses a rush-current limiting circuit for limiting the charging current of a series-connected smoothing capacitor when voltage is connected and in the event of voltage fluctuations. The load current of the load buffered by the smoothing capacitor does not flow through the current limiting circuit.
Patent Abstracts of Japan E-811, Aug. 24, 1989, Vol. 13/No. 383, 1-133567 (A), discloses a further current limiting circuit which is connected in series with a smoothing capacitor. Again, the load current of a load connected downstream does not flow through the current limiting circuit.
FIG. 1 illustrates a conventional electrical power supply EPS with an input voltage Vi and a regulated DC output voltage Vo. The input voltage Vi is either a DC voltage or, preferably, an unsmoothed voltage produced at the output of a rectifier (not illustrated). The electrical power supply EPS contains a switching regulator SR0. An inrush current limiter ICL0 is connected upstream of said switching regulator SR0, between its terminals T1 and T3. The inrush current limiter ICL0 has a current limiting circuit CL0 connected in series with a support capacitor C0, which serves the particular purpose of bridging momentary failures of the input voltage Vi applied to the input terminals T4 and T5. The support capacitor C0 generally has a high capacitance. Electrolytic capacitors are advantageously suitable for this purpose.
The support capacitor C0 is in the uncharged state before the electrical power supply EPS is switched on. During the switch-on process, the capacitor C0 is charged by means of the input voltage Vi. The charging current required for charging the support capacitor C0 is prevented from reaching excessively high peak values by the current limiting circuit CL0, which is connected in series with the support capacitor C0. As soon as the support capacitor C0 has reached its charged state, the current limiting circuit CL0 assumes a low internal resistance. The load current flowing through the input terminals T4 and T5 to the terminals T1 and T3 of the switching regulator SR0 does not pass through the current limiting circuit CL0, nor does it produce any further losses there.
A disadvantage of the arrangement is that overall the inrush current limiter ICL0 has an increased impedance, or an increased internal resistance, since the current limiting circuit CL0 is connected in series with the support capacitor C0. This increased impedance of the support capacitor C0 impairs the smoothing properties of this capacitor.
It is particularly disadvantageous that the support capacitor C0 is accordingly less able to buffer superposed interference voltages caused by the switching regulator SR0, such as so-called ripple voltages. These voltages are thus passed on to a greater extent to the input terminals T4, T5 of the electrical power supply EPS. Consequently, relatively costly radiointerference suppression may be needed to combat the superposed interference voltages that are passed on, to an increased extent, to the input terminals T4,T5.